This invention relates to systems for identifying objects on a remote basis. The system includes a reader for providing a simple and reliable identification of an object which is a considerable distance away by detecting a unique sequence of signal cycles identifying the object.
As commerce becomes increasingly complex, the volume of moving articles and vehicles requiring individual identification increases. For example, containers holding goods are stacked on merchant ships. When the merchant ships reach a destination port, only a portion of the containers need to be unloaded, the remaining ones staying on the ship for subsequent destination ports. It is thus desirable to identify the containers remotely, as they are being loaded or unloaded.
Systems of this nature have been developed. Such systems include a reader, displaced from the object, for transmitting a signal which interrogates an electronic transponder tag on the object. The tag has an identifying code which is unique to the object being interrogated. This code is represented by a sequence of binary 1's and 0's. Each binary 1 and 0 in this sequence is converted to a plurality of signal cycles having a predetermined periodicity which are transmitted to the reader. The signal cycles in each plurality are made up of portions having two different frequencies in a particular pattern, one pattern to identify a binary "1" and another pattern to identify a binary "0". The antenna at the reader picks up a signal reflected from the transponder at the object which contains the unique signals from the object.
In some applications, systems of this type employ a single reader to read multiplexed signals from several antennas. For example, it is sometimes desirable to read data from electronic tags on passing railroad cars. These cars often have transponder tags only on one side of the car. However, since railroad cars commonly get turned around, one never knows on which side of the track the tag will be. Therefore one must have an antenna on both sides of the track.
One simple solution, of course, would be to have two antennas and two readers, one on each side of the track. Alternatively, of course, one can put tags on both sides of the railroad car. Both of these are obviously wasteful solutions. A better solution is to have a single tag on each car with antennas on opposite sides of the track, both multiplexed to a single reader.
The difficulty with this solution is that the multiplexing of the two antenna inputs to a single reader must be extremely fast. The reader must be able to determine quickly that there is no signal from one antenna, and still have time to check the antenna on the other side of the rapidly passing railroad car for a signal from a tag on the opposite side.
With the readers which were available prior to this invention, the only way the presence or absence of a tag signal could be ascertained was to completely read all of the information contained in the tag and make computations to see if it was valid information. If invalid, the reader became free to be switched to another antenna. If this technique were employed with fast moving railroad cars, it is likely that by the time the computer ascertained that there was no tag present on one side, and switched to receive signals from the antenna on the opposite side of the car, the tag, if any, on the second side would have passed beyond the antenna's range.
These same systems are also used to identify automobiles in toll booth lanes for automatic toll collection. Where a single reader is to be multiplexed to several antennas in several lanes approaching the toll booth, if the absence of a car cannot be detected quickly, more readers must be used for fewer lanes, perhaps even one reader for each lane.